I apologize if this is confusing, so feel free to ask for clarifications.

I'm getting close to publishing the first book of a series that was originally the first few chapters of one book I decided to expand (for the most part, I still consider the series one giant book). The "original book" follows the life of a boy as told from the POV of people who had a major influence on his life. The first book's protagonist is one of these POV with respective character/plot arc. Obviously, this arc is what I want to tell people the book is about/promote it as, however, is it worth mentioning it's part of a longer story -- especially since the series MC and protagonist aren't the same person? Or is it something for the reader to figure out on their own (i.e. second book is irrelevant if they can't get through the first)?